Handle nvp default l3 gw service misconfiguration appropriately

Bug 1185885

Avoid returning an unrelated, backend-specific, error message to
the user, and ensure the logical router is removed from the
NVP backend.

Change-Id: I5f7b88480dab80a2138486e612e144b971691cf5
Merge "Handle nvp default l3 gw service misconfiguration appropriately"